1. Field of the Invention
The present invention relates to a luminous composition and an electroluminescent (EL) device comprising the same. In particular, the present invention relates to a luminous composition which is useful for the formation of a luminous layer of a dispersion-type organic EL device, has improved adhesion to the transparent electrode of the EL device, and suffers from no or little deterioration of its pot life and moisture resistance, and a dispersion-type organic EL device having a luminous layer comprising such a luminous composition.
2. Description of Prior Art
In general, a dispersion-type organic electroluminescent device (hereinafter referred to as an "EL device") comprises a back electrode, a reflective insulation layer, a luminous layer and a transparent electrode, which are laminated in this order.
The luminous layer generally comprises a composition containing, as binder resins, various fluoropolymers (for example, homo- or copolymers of vinylidene fluoride, etc.), and recently graft polymers comprising such fluoropolymers to which cyanoethylated acrylic monomers are grafted (see, for example, JP-A-7-114988 and JP-A-8-134149).
However, a luminous layer, which is formed from a composition containing such fluoropolymers or graft polymers, a fluorescent material powder and an organic solvent, has low adhesion to a transparent electrode. Thus, the luminous layer tends to be peeled off from the transparent electrode during a cutting step, and the like. Therefore, a ratio of inferior devices increases. Furthermore, the EL devices have less flexing durability in use.
To improve the adhesion between the luminous layer and transparent electrode, following measures have been employed:
(1) lowering the glass transition temperature (Tg) of a used binder resin to soften the luminous layer, and PA1 (2) treating the surface of the transparent electrode. PA1 (A) at least one polymer selected from the group consisting of a fluoropolymer and a graft polymer which comprises a fluoropolymer to which a cyanoethylated acrylic monomer is grafted, PA1 (B) an alkoxysilane compound having a primary amine group, PA1 (C) an alcohol having 1 to 4 carbon atoms, PA1 (D) a fluorescent material powder, and PA1 (E) an organic solvent. PA1 (i) A mixture of a dielectric binder resin and an inorganic dielectric material powder (for example, titanium oxide, barium titanate, barium zirconate, barium stannate, strontium titanate, etc.) in an organic solvent is applied on one surface of a back electrode, and heated and dried to form a reflective insulation layer. Then, the luminous composition of the present invention is applied on the reflective insulation layer, and heated and dried to form a luminous layer. After that, a transparent electrode is laminated on the luminous layer and heat pressed. PA1 (ii) Alternatively, a reflective insulation layer is formed on a back electrode while a luminous layer is formed on a transparent electrode. Then, the electrodes are laminated with the reflective insulation layer and the luminous layer facing each other, and heat pressed.
In the case of the measure (1), the moisture resistance of the luminous layer deteriorates, and in turn, the deterioration of the luminous devices may be accelerated. In the case of the measure (2), the production cost of the EL devices increases.
In general, the adhesion between layers can be improved with the addition of silane coupling agents to one or both of the layers. However, when alkoxysilane compounds having a primary amino group (--NH.sub.2) are compounded in the above fluoropolymers, the both materials react each other to form a gel, and thus the pot life after compounding greatly shortens. It is necessary for a practical pot life of a composition to meet a required time from compounding to the use in the production of EL devices and coating steps (using a doctor blade or by screen printing). In general, the composition should not be gelled about one day under storage conditions at a constant temperature of 40.degree. C.